In sliding gate gravity outlets it has been found that careful attention to the flatness of the gate and its supporting structure will produce satisfactory sealing at the leading edge and the sides of the gate.
The trailing edge of the gate passes through a slot in the outlet pan which must be of such size as to allow the gate to slide without binding under the minimum tolerance conditions. This clearance is increased by the allowable manufacturing tolerances. This clearance can amount to as much as six square inches of opening.
The Association of American Railroads (AAR) has issued a specification on gravity outlets which includes an acceptance test which provides that the outlet is acceptable if it will retain any part of fifty (50) gallons of water for a period of three (3) minutes.
With the slot condition which exists such outlets will not pass this acceptance test without a seal which closes off this slot.
This open slot also allows considerable escape of fine powders such as cement and flour as dust when handling such materials.
Several types of seals, such as the "P" seal described in U.S. Pat. No. 3,958,514 and a blade seal used on a gravity outlet have been used.
While use of these seals have allowed the outlets to meet the water test and reduce dusting, they have the disadvantage of short life, increased drag on the gate and high cost.